Using glRotate and glTranslate with collision detection - c++

Question

Say I use glRotate to translate the current view based on some arbitrary user input (i.e, if key left is pressed then rtri+=2.5f)
glRotatef(rtri,0.0f,1.0f,0.0f);
Then I draw the triangle in the rotated position:
glBegin(GL_TRIANGLES); // Drawing Using Triangles
glVertex3f( 0.0f, 1.0f, 0.0f); // Top
glVertex3f(-1.0f,-1.0f, 0.0f); // Bottom Left
glVertex3f( 1.0f,-1.0f, 0.0f); // Bottom Right
glEnd(); // Finished Drawing The Triangle
How do I get the resulting translated vertexes for use in collision detection? Or will I have to manually apply the transform myself and thus doubling up the work?
The reason I ask is that I wouldn't mind implementing display lists.

Answer 1

The objects you use for collision detection are usually not the objects you use for display. They are usually simpler and faster.
So yes, the way to do it is to maintain the transformation you're using manually but you wouldn't be doubling up the work because the objects are different.

Answer 2

Your game loop should look like (with c++ syntax) :
void Scene::Draw()
{
this->setClearColor(0.0f, 0.0f, 0.0f);
for(std::vector<GameObject*>::iterator it = this->begin(); it != this->end(); ++it)
{
this->updateColliders(it);
glPushMatrix();
glRotatef(it->rotation.angle, it->rotation.x, it->rotation.y, it->rotation.z);
glTranslatef(it->position.x, it->position.y, it->position.z);
glScalef(it->scale.x, it->scale.y, it->scale.z);
it->Draw();
glPopMatrix();
}
this->runNextFrame(this->Draw, Scene::MAX_FPS);
}
So, for instance, if i use a basic box collider with a cube the draw method will :
Fill the screen with a black color (rgb : (0,0,0))
For each object
Compute the collisions with position and size informations
Save the actual ModelView matrix state
Transform the ModelView matrix (rotate, translate, scale)
Draw the cube
Restore the ModelView matrix state
Check the FPS and run the next frame at the right time
** The class Scene inherits from the vector class
I hope it will help ! :)